Hybrid vehicles, such as plug-in hybrid vehicles, may have two modes of operation: an engine-off mode and an engine-on mode. While in the engine-off mode, power to operate the vehicle may be supplied by stored electrical energy. While in the engine-on mode, the vehicle may operate using engine power. By switching between electrical and engine power sources, engine operation times may be reduced, thereby reducing overall carbon emissions from the vehicle. However, shorter engine operation times may lead to insufficient purging of fuel vapors from the vehicle's emission control system. Additionally, refueling and emission control system leak detection operations that are dependent on pressures and vacuums generated during engine operation may also be affected by the shorter engine operation times in hybrid vehicles.
Various strategies have been developed to address fuel vapor control and management in hybrid vehicle systems. Example approaches include separating storage of refueling vapors from storage of diurnal vapors by adding a fuel tank isolation valve (FTIV) between a fuel tank and a fuel vapor retaining canister, and allowing refueling vapors to the canister during refueling events, and engine-on purging methods. The separation of diurnal and refueling vapors allows a pressure to be generated in the fuel tank, while application of alternative vacuum sources allows a vacuum to be generated in the canister.
One example approach for fuel vapor management is shown by Ito el al. in U.S. Pat. No. 6,557,401. Therein, leak detection of fuel vapor recovery system components is performed in two stages. First the fuel tank is sealed and a change in fuel tank pressure is measured over time. Next, a vacuum is applied to the canister. Presence of leaks is determined based on changes in the fuel tank pressure and the canister pressure over time.
Another example approach is shown by Takagi et al. in U.S. Pat. No. 6,761,154. Therein, leak detection is performed by operating a pump to apply a vacuum on the carbon canister, followed by monitoring a change in canister pressure over time. A valve disposed between the fuel tank and the carbon canister is then opened to apply the vacuum to the fuel tank, followed by monitoring a change in fuel tank pressure over time. Presence of leaks may be determined based on changes in canister pressure and fuel tank pressure over time
However, the inventors herein have recognized potential issues with these approaches. As one example, these approaches fail to address the transitory nature of pressure and vacuum accumulation in a hybrid vehicle system due to infrequent and irregular engine operation. For example, the shorter duration of engine operation in hybrid vehicles may lead to lower amounts of vacuum being generated during an engine-on mode, such that insufficient vacuum may be present in the fuel tank for a subsequent leak detection operation. As a result, there may not be sufficient pressure and/or vacuum for detecting leaks in both the fuel tank and the carbon canister. Since leak detection in the fuel tank in the above approaches is tied to leak detection in the carbon canister, insufficient pressure and/or vacuum may lead to incomplete fuel vapor recovery system leak detection. Also, operation of an external dedicated pump to generate vacuum and/or pressure for leak detection may increase system cost and power consumption.
The above issues may be at least partly addressed by a method of monitoring a vehicle fuel vapor recovery system coupled to an engine intake, said fuel vapor recovery system including a fuel tank coupled to a canister via a fuel tank isolation valve, the canister coupled to the engine intake via a canister purge valve, the canister further coupled to a vacuum accumulator via a vacuum accumulator valve. The method may comprise, under a first condition, applying a pressure on the fuel tank before applying a pressure on the canister; and under a second condition, applying a pressure on the canister before applying a pressure on the fuel tank; and under the first or second condition, indicating degradation based on a change in a fuel vapor recovery system pressure value upon pressure application.
In one example, a fuel vapor recovery system for a hybrid vehicle may comprise a fuel tank coupled to fuel vapor retaining device (such as a carbon canister) via a fuel tank isolation valve (FTIV). The canister may be coupled to the engine intake via a canister purge valve (CPV). The canister may be further coupled to a vacuum accumulator via a vacuum accumulator valve (VAV). As such, the FTIV may be maintained in a closed state during vehicle operation and may be selectively opened during refueling and diurnal vapor purging conditions. By maintaining the FTIV closed, the fuel vapor circuit may be divided into a canister side and a fuel tank side. Refueling vapors may be retained in the canister on the canister side of the circuit while diurnal vapors may be retained in the fuel tank on the fuel tank side of the circuit.
A first pressure sensor may be coupled to the fuel tank to estimate a pressure of the fuel tank side of the circuit, while a second pressure sensor may be coupled to the canister to estimate a pressure of the canister side of the circuit. Based on input from various sensors, such as the pressure sensors, and further based on vehicle operating conditions, a controller may adjust various actuators, such as the VAV, the CPV, the FTIV, and a canister vent valve (CVV), to enable fuel tank refueling, purging of stored fuel vapors, and leak detection in the fuel vapor recovery system.
In one example, during leak detection, an order of monitoring components of the fuel vapor recovery system may be adjusted based on an amount of pressure and/or vacuum available for the leak detection in either of the carbon canister or the fuel tank. For example, if sufficient pressure and/or vacuum is not available in the fuel tank for leak detection, vacuum from the vacuum accumulator may be applied to the carbon canister by opening the VAV. In this case, first the carbon canister may be checked for leaks, then the operation of the FTIV may be monitored, and then the fuel tank may be tested for leaks. In comparison, when the fuel tank does have sufficient pressure and/or vacuum for leak detection, the order of leak detection may be changed, wherein first the fuel tank may be tested for leaks, then the operation of the FTIV may be determined, and finally the carbon canister may be checked for leaks.
In one example, leak detection may involve monitoring a change in fuel tank pressure and/or a canister pressure over time. For example, leaks may be identified based on a rate of change in pressure during the vacuum/pressure application, or based on difference before and after vacuum/pressure application. In another example, leak detection may be based on temperature and pressure changes in the fuel tank.
In this way, by adjusting an order of application of vacuum and/or pressure on fuel vapor recovery system components based on availability of vacuum and/or pressure, leak detection may be performed on all the components of the system even when the duration of the engine-on operation varies in the hybrid vehicle. Additionally, leak detection in the components may be decoupled from each other based on the amount of pressure and/or vacuum available. By decoupling leak detection in a first component, such as the fuel tank, from leak detection in a second component, such as the canister, a more robust leak detection routine may be possible.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.